1. Field of the Invention
This invention relates to a polyimide multilayer wiring substrate, wherein polyimide resin is employed as interlaminar insulation when multilayer wiring layer is formed on a ceramic board, and a method for manufacturing the same.
2. Description of the Related Art
Multilayer printed wiring substrates have conventionally been used as wiring substrates for carrying LSI chips thereon. A multilayer printed wiring substrate is constructed using a copper plated layered plate as a core member and using a prepreg as a bonding agent for the core member, and the core members and the prepregs are layered alternately, integrally with each other using a heat press. Electrical connection between the layered plates is established by forming, after the core members and prepreg are formed into a single structure, through-holes by drilling and thereafter plating the inner walls of the through-holes with copper.
Recently, multilayer wiring substrates, in which a polyimide resin is employed as interlaminar insulation when multilayer wiring layer is formed on a ceramic board, have been used for wiring substrates for use in large computers, which require a wiring density higher than that of multilayer printed wiring layers. For manufacturing a polyimide-ceramic multilayer wiring substrate, a polyimide multilayer wiring layer formed by repeating a series of steps comprising a polyimide resin insulation layer forming step, in which polyimide precursor varnish is applied to a ceramic board and dried to form a coated film, and viaholes are formed in said film, and a wiring layer forming step, in which photolithography, vacuum deposition and plating are used.
Aside from the above-mentioned method, there is a method in which wiring patterns are first formed on polyimide sheets and then the polyimide sheets are successively positioned and layered under pressure on a ceramic board to form a multilayer wiring substrate. Since a signal layer is formed individually, sheets free from defects can be selectively laminated, resulting in improved manufacturing yield as compared to the successive layering method as described above.
In the multilayer printed wiring substrate described above, since electrical connection between layered plates is established by means of through-holes formed therein by drilling, fine through-holes can not be formed, and hence the number of wirings which can be formed between the through-holes is limited. Further, one through-hole is required for the connection in one layered plate, and as the number of layers increases, the signal wiring capacity decreases, thereby leading to a drawback that it is difficult to form a multilayer printed wiring substrate having a high wiring density.
Meanwhile, a polyimide-ceramic multilayer wiring substrate which has been developed recently in order to offset the drawback of a conventional multilayer printed wiring substrate described above, requires repetition of the steps including a step of applying polyimide precursor varnish to a ceramic board , a drying step, a step of forming viaholes and a curing step, in which the number of repetition is equal to the number of polyimide insulation layers. Consequently, a very long period of time is required for the layering step for a multilayer wiring substrate. Further, since the step of forming a polyimide insulation layer is performed repetitively, there is another drawback that thermal stress in the curing step is imparted many times to the polyimide resin at lower layers of the multilayer wiring substrate layer, which deteriorates the polyimide resin. The polyimide multilayer wiring substrate has a further drawback that, since they are formed by a sequential layering method, it is difficult to enhance manufacturing yield.
Also the method for layering sheets one by one, which has been developed as a method to enhance the manufacturing yield, still has the drawbacks that, since layering is successively performed under pressure for each layer, as the number of layers increases, the thermal stress imparted to the polyimide resin of lower layers increases and the deterioration of the polyimide resin occurs mainly at an earlier stage and that a great number of days are required to manufacture a substrate.
In order to overcome the drawbacks described above, a yet further method has been proposed wherein a polyimide multilayer wiring substrate is formed by successively stacking a plurality of blocks each consisting of a layered structure including a plurality of wiring layers and dissolving, each time one block is stacked, a base material such as aluminum using a solution such as hydrochloric acid as occasion calls. The method, however, still has a problem that wirings are invaded by the liquid for dissolving the base material.